28 
AV. F. rUKCELL. 
iippearaiice, creates the impression that it \va^ designed and 
is about to grow outwards, aud one is perhaps justified in 
asking why, if the saccules were originally derived from 
tracliea-like invaginations, the two walls of a saccule do 
not appear simultaneously as we should expect from a fold 
originally designed to grow inwards ? I do not, however, 
wish to attach too much importance to this point, as it is very 
difficult to ascertain with certainty, and would not even then 
constitute a clear proof either way. 
Passing to the physiological side of the question, one 
benefit derived from the sinking of the gill-leaves into the 
appendage and of the latter into the body would, of course, 
as Kingsley says, be protection from the increased wear and 
tear incidental to terrestrial motion. I’he delicate gill-leaves 
wdth the three unattached edges would be very liable to 
injury when deprived of liquid support, while a lung-septum, 
having only one unattached edge, is perfectly secure. At 
first, no doubt, the gill-leaves would be very sensitive to 
evaporation, and the cavities between their basal portions in 
the intermediate stage (fig. 11) in Kingsley, '85) may have 
formed convenient reservoirs for retaining water to moisten 
the respiratory surfaces during terrestrial excursions. 
Various other theories have been suggested by different 
authors (Milne-Edwards, ’72, p. oG ; Kay Lankester, ’81, ’85a 
and ’85b; iMacLeod, ’82 and ’84; Jjaurie, ’92 and ’93) to explain 
how gill-books like those of Limulus, may have been con- 
verted into lung-books, but none of them correspond exactly 
to the embrvological facts, so I shall not consider them 
further in this pa]>er. 
Later development of the pulmonary saccules. — 1 resume the 
descripition at the 0-6-iun’ow stage represented in figs. 
18-1 8 b and 1G-1 Gk. 'I'lie interior of the appendage has 
become nearly filled out by the ingi-owing saccules, which 
push befoi'e them the intra-appendicular part of the coelom 
and ultimately occiqty its place. They continue to grow 
till the anterior side of the aj)])endage is reached. The oldest 
saccules are still the longest, but are exceeded in breadth by 
